Method and apparatus providing single cable bi-directional triggering between instruments

Abstract

System and apparatus enabling the use of a single cable to communicate triggering information between each of a plurality of signal acquisition devices and, illustratively, an external trigger control unit. A combined trigger signal is produced only when each trigger condition of each signal acquisition device is true. Thus, all of the signal acquisition devices will be triggered substantially simultaneously.

1. A system for triggering a plurality of test and measurement instruments substantially simultaneously, comprising:
 a first test and measurement instrument having an input for receiving a signal under test, a trigger circuit for developing a trigger signal in response to said signal under test, and a transceiver for developing a trigger enable signal in response to said trigger signal and receiving a combined trigger signal; 
 a second test and measurement instrument having an input for receiving a signal under test, a trigger circuit for developing a trigger signal in response to said signal under test, and a transceiver for developing a trigger enable signal in response to said trigger signal and receiving combined trigger signal; and 
 circuitry for logically combining said trigger enable signals of said first and second test and measurement instruments to generate said combined trigger signal, the circuitry for combining having a first and second transceivers for receiving said trigger enable signals and transmitting said combined trigger signal; 
 wherein each of said test and measurement instruments is coupled to said circuitry for combining via a cable, said trigger enable signal and said combined trigger signal being conveyed in mutually opposite directions through said cable; and 
 said first and second test and measurement instruments acquire data samples of said signals under test in response to said combined trigger signal. 
 
   
   
     2. The system of  claim 1 , wherein said transceivers comprise:
 a series combination of a variable impedance device, a switch and a constant current source; wherein: 
 a junction of said variable impedance device and said switch is adapted to transmit said trigger enable signal. 
 
   
   
     3. The system of  claim 2 , wherein an output terminal of said variable impedance device is monitored to receive said combined trigger signal. 
   
   
     4. Apparatus for use in a test and measurement instrument, comprising:
 an event decoder, for monitoring at least one input signal to determine whether a logical triggering event has occurred, and generating a trigger enable signal in response thereto; 
 a terminal for receiving a conductor, said conductor coupling signals between said apparatus and an external device, said conductor conveying said trigger enable signal and a trigger signal in mutually opposite directions; and 
 a transceiver, coupled to said terminal, for transmitting said trigger enable signal and for receiving said trigger signal. 
 
   
   
     5. The apparatus of  claim 4 , wherein said test and measurement instrument further comprises an acquisition unit, for acquiring a plurality of said data samples from at least one input signal in response to said trigger signal. 
   
   
     6. The apparatus of  claim 4 , wherein said transceiver comprises:
 a series combination of a variable impedance device, a switch and a constant current source; wherein 
 a junction of said variable impedance device and said switch is adapted to transmit said trigger enable signal. 
 
   
   
     7. The apparatus of  claim 6 , wherein:
 an output terminal of said variable impedance device is monitored to receive said trigger signal. 
 
   
   
     8. The apparatus of  claim 6 , wherein:
 said variable impedance device comprises a transistor. 
 
   
   
     9. The apparatus of  claim 4 , wherein:
 said apparatus is used in each of a plurality of test and measurement instruments, each of said plurality of test and measurement instruments using its respective transceiver to transmit a respective trigger enable signal and to receive said trigger signal. 
 
   
   
     10. The apparatus of  claim 9 , wherein:
 said external device is a trigger controller; and 
 each transceiver of each of said test and measurement instruments communicates with a corresponding transceiver in said trigger controller, said trigger controller logically combining said respective trigger enable signals to produce said trigger signal.